It's only another tool in the diagnostic toolkit. My presentation is, um, "non-classical" to say the least (intentional tremor being, perhaps, the weirdest part of it) and arriving at a diagnosis of Parkinson's disease pretty much involved ruling out everything else that might have been causing my symptoms (which is a rather long process, all things considered). I've met dozens of people who I could have spotted as Parkinson's sufferers two blocks away at night during a blackout, and a few who, like me, seem to have a bespoke version that "isn't the other things" and responds to treatment for Parkinsonism. Well, as well as anything does, at least - meds are always a pretty blunt weapon and can't exactly track the body's own chemistry, so there will always be good periods, and under/over-medicated periods as well. When it comes to neuropathy - or normal functioning, for that matter - we're not quite stumbling around in the dark, but there's still one hell of a long way to go before we can say that we understand much about the machine.